Electronic parts such as CPUs or semiconductor chips used for various electronic devices such as computers generate a large amount of heat during operation. Such electronic devices are usually designed to operate at room temperature. Accordingly, when heat generated during the operation of an electronic device is not effectively cooled down, the performance of the electronic device is severely deteriorated and, in some cases, the electronic device itself may be damaged.
As a method of cooling down heat generated by various electronic parts, many approaches have been developed such as a heat conduction method using a heat sink, a method of using natural convection or radiation of air, a force convection method using a fan, a method using circulation of liquid, or a submerged cooling method.
However, as nowadays many released electronic products are made slim, an installation distance between electronic parts generating heat during operation is continuously decreased so that heat is not appropriately cooled down. Also, since the heat load of electronic parts has continuously increased due to the high integration and high performance of the electronic parts, the above-described cooling methods are not able to effectively cool down the electronic parts.
As a new technology to solve the above problems, a phase change heat transport system which can cool down an electronic part having a high heat load density per unit has been introduced. A thermosyphon system and a cylindrical heat pipe system are examples of the phase change heat transport system.
According to the thermosyphon system, cooling is achieved using a natural circulation method via a liquid-vapor phase change and a specific gravity difference of working fluid. In a conventional cylindrical heat pipe, cooling is obtained by circulating the working fluid using a capillary pumping force generated by a sintered wick installed in an interior wall of a pipe.
However, in the thermosyphon system, a condenser section must be located higher than an evaporator section. Although this problem is less severe in the case of the heat pipe of the thermosyphon system, a heat transport ability of the heat pipe is quite deteriorated when the condenser section is located lower than the evaporator section in the field of gravity. Accordingly, since there is a limitation in the positional relationship between the constituent elements in the above two systems, this limitation prevents electronic devices employing the above cooling systems from being made slim.
Also, since vapor and liquid flow in opposite directions in a linear pipe of the thermosyphon or the cylindrical heat pipe, the vapor and the liquid may be mixed in the middle of the pipe. Another problem is that the mixture may make the amount of heat actually transported less than that that can be ideally transported.
A loop heat pipe (LHP) system has been suggested as an ideal heat transport system which can solve these problems, that is, the positional limitation and the mixture between the vapor and liquid. The LHP system is a sort of a capillary pumped loop heat pipe (CLP) technology developed by the NASA, U.S.A., to cool down a large amount of heat generated from communications equipment or electronic equipment used for artificial satellites.
Korean Patent No. 671041 entitled “Loop Heat Pipe” discloses a technology about a compact loop heat pipe system. FIG. 1 illustrates a loop heat pipe system 110 according to this conventional technology. The conventional loop heat pipe system 110 includes a condenser 112, an evaporator 114, a vapor line 116, and a liquid line 118, which form a loop. The vapor line 116 and the liquid line 118 are connected between the condenser 112 and the evaporator 114. In the loop heat pipe system 110, a sintered wick 120 is installed only in the evaporator 114 unlike the conventional linear heat pipe of FIG. 1.
In the present specification, the loop heat pipe is referred to as a loop heat pipe system and both terms have the same meaning. Also, the evaporator and the condenser, respectively, have the same meanings as the evaporator section and the condenser section.
The loop heat pipe system 110 operates in the following manner. Heat is applied to a heating plate 122 which is the bottom portion of the evaporator 114 in which the sintered wick 120 is inserted. At that point the sintered wick 120 is saturated with the liquid phase of working fluid due because the heat transported to the sintered wick 120 contacting the heat plate 122. And the applied heat vaporizes the working fluid so that the phase of the working fluid is changed to a vapor state. The vapor is moved toward the condenser 112 along the vapor line 116 connected to a side of the evaporator 114. As the vapor passes through the condenser 112, heat is dissipated externally so that the vapor is liquefied. The liquefied working fluid is moved toward the evaporator 114 along the liquid line 118 at a side of the condenser 112. The above-described process is repeated so that the heat source can be cooled down.
In the evaporation of the working fluid permeated in the sintered wick 120, referring to FIG. 3 that is a cross-sectional view taken along line III-III of FIG. 2 and FIG. 4 showing the sintered wick 120 of FIG. 2 rotated by 180° for the convenience of explanation, a surface 126 of the sintered wick 120 facing the heating plate 122 includes a contact surface 126b contacting the heating plate 122 and a plurality of micro-channels 126a working as a passage of the generated vapor. Accordingly, the sintered wick 120 receives heat via the contact surface 126b contacting the heating plate 120 so that the received heat makes the working fluid permeated in the sintered wick 120 evaporate. The generated vapor is moved toward the condenser 112 along the vapor line 116 connected to a side of the evaporating portion 114 through the micro-channels 126a formed in the surface 126 facing the sintered wick 120.
On the other hand, the performance of an evaporator taking heat from a heat source like an electronic part is determined according to how well the heat transported from the heat source to a heating plate is transported to a sintered wick. In this connection, contact conductance is a factor directly affecting the heat transport between the heat source and the heating plate.
The contact conductance is related to the thermal resistance generated when a metal has a surface contact with another metal and heat transport occurs between the metals. The contact conductance is proportional to the contact area between the two metals. That is, as the contact area increases, the contact conductance increases, and as the contact conductance increases, heat transport is generated further.
However, in a method of manufacturing an evaporator employed in the conventional loop heat pipe system, since the sintered wick having the channels that are the passage of vapor is separately manufactured and then coupled to the heating plate, the manufacturing process is complicated and the state of a coupling surface is not satisfactory. Prior to a description about disadvantages, the conventional method of coupling the sintered wick to the heating plate includes a simple pressing method and a metal coupling method.
According to the simple pressing method, a contact portion of a previously manufactured metal sintered wick is processed/non-processed and a predetermined load is applied to the contact portion so that the sintered wick and the heating plate contact each other and are coupled to each other. In the metal coupling method, metal powder is previously sintered to form a sintered wick and a plurality of channels that are the passage of vapor are formed. After the sintered wick with the channels contacts the heating plate, the sintered wick and the heating plate are sintered again so that metallic coupling is formed between the sintered wick and the heating plate.
On the other hand, in the metal coupling method used for a conventional cylindrical heat pipe with sintered wick, when metal powder is sintered, an auxiliary jig is used to allow the metal powder to contact a heating boundary surface. Then, the metal powder is sintered and coupled to an inner wall of a metal pipe. For a cylindrical heat pipe, since a vapor generation position is not a boundary surface between metal and the sintered wick, but a surface of the sintered wick, a separate vapor passage is not needed so that a method of directly installing metal power on a metal pipe and sintering the metal powder may be used.
However, in order to manufacture the conventional evaporator used for a loop heat pipe system, since the sintered wick with the vapor passage is coupled to the heating plate and vapor is generated from the boundary surface between the sintered wick and the heating plate, the method of directly installing the metal powder to the heating plate and sintering the metal power as in the above-described cylindrical heat pipe cannot be used. Rather, a metal coupling method in which the vapor passage must be formed in a surface of the sintered wick, the side surface is installed on a metal heating plate, and the sintered wick is sintered needs to be used.
Consequently, in order to manufacture the conventional evaporator used for a loop heat pipe system, metal powder is sintered and the channels are formed in the sintered metal powder. Then, the sintered wick with the channels is coupled to the heating plate in the simple pressing method or metal coupling method. Accordingly, a manufacturing process is complicated and a manufacturing cost is high. In particular, since the coupling between the heating plate and the sintered wick is not satisfactory, a value of contact conductance is lowered so that heat transport is not effectively and sufficiently performed. Also, according to the metal coupling method of primarily sintering the metal powder as sintered wick first and then sintering the sintered wick again), changes in the shape, size, porosity, and permeability of the sintered wick due to the fact that the contraction rate of the sintered wick cannot be avoided.
Also, according to the conventional method of manufacturing an evaporator for a loop heat pipe system, the manufactured evaporator has a relatively low contact conductance value as the contact area between the sintered wick and the heating plate is reduced by the existence of the vapor passage (the micro-channels). That is, referring to FIG. 3 showing a cross-section in which the heating plate 122 and the sintered wick 120 having the vapor passage (the micro-channels 126a) are coupled to each other, viewed in a direction making 90° with the direction of the cross section of FIG. 2, the size of the contact surface 126b where the sintered wick 120 and the heating plate 122 contact each other is reduced by the vapor passage so that the amount of heat cannot be sufficiently transported.